[unreadable] [unreadable] Agrin stimulates the clustering and tyrosine phosphorylation of the acetylcholine receptor (AChR) in cultured myotubes. These data have led to the hypothesis that tyrosine phosphorylation of the AChR may be a critical, and possibly a requisite step to cluster AChRs and other postsynaptic proteins at neuromuscular synapses. Using site-directed mutagenesis and homologous recombination in embryonic stem (ES) cells, I have generated mutant mice that bear mutations in tyrosine residues within the large intracellular loop of AChR-[unreadable]. My preliminary results indicate that tyrosine phosphorylation of AChR-[unreadable], while not necessary to form synapses per se, is required to form synapses of normal AChR size, density and morphology. The manner by which AChR-[unreadable] tyrosine phosphorylation leads to changes in AChR architecture, however, is unknown. Further, the functional effects of these synaptic perturbations on neuromuscular transmission are not understood. Using molecular biology, cell biology, microscopy and electrophysiology I propose a set of experiments to address these questions in hopes of further understanding the role of AChR-[unreadable] tyrosine phosphorylation in synapse formation. [unreadable] [unreadable]